1. Field of the Invention
The present invention relates to a code synchronization decision circuit of a Viterbi decoder for deciding as to whether or not code synchronization is established in the Viterbi decoder which is employed in a digital transmission system.
2. Description of Related Art
FIG. 5 is a timing chart showing timings of code synchronization decision operation by a conventional code synchronization decision circuit of a Viterbi decoder. As illustrated in FIG. 5, the conventional code synchronization decision circuit of the Viterbi decoder counts the number of errors of symbols over a fixed symbol measuring period, and compares the count value with a predetermined error number (called threshold value from now on). As a result, if the count value does not exceed the threshold value, a decision is made that the code synchronization is established, and otherwise a decision is made that the code synchronization is not established. If the code synchronization is not established, the Viterbi decoder cannot decode a convolutional code correctly. Accordingly, when the code synchronization decision circuit decides that the code synchronization is not established, it outputs a signal notifying of that.
Generally, the conventional code synchronization decision circuit continues counting the number of errors for each symbol measuring period by keeping operation of both the error counter and symbol counter for deciding the code synchronization even during a time period in which a re-encoder that re-encodes the decoded output of the Viterbi decoder produces a signal inappropriate for counting the number of errors immediately after the start of operation, or immediately after the detection of a slip signal notifying that the code synchronization is lost.
As documents disclosing a conventional code synchronization decision circuit of the Viterbi decoder, Japanese patent application laid-open Nos. 5-206872/1993 and 6-260945/1994 are known.
The conventional code synchronization decision circuit of the Viterbi decoder with such an arrangement has the following problems. First, it is difficult for transmission line characteristics a user considered to be correctly reflected on settings when the user establishes the threshold value and the number of symbols counted by the symbol counter. Second, rather much power is wasted by the idle operation during the time period in which the inappropriate signal is supplied for the error counter to count the number of errors.